Even so-called inexpensive, portable microscopes are relatively expensive and bulky. Those microscopes considered to be in the less expensive range are often of lower power and omit many of the features which are found in better instruments. Focussing usually requires a special mechanism; a source of light is generally required, and the devices tend to be fragile. In addition, such devices often require specially sized and shaped slides and have no provision for carrying spare slides or optical elements for use with the microscope.